1. Field of the Invention
The present invention relates to an optical apparatus which enables high-speed measurement of a real shape of a wafer for detection of a flatness, a thickness and profiles of both surfaces.
2. Brief Description of the Prior Art
A flatness or thickness variation of a wafer has been measured so far by either an optical method using interference fringes or a physical method using displacement sensors which scan both surfaces of the wafer.
In the conventional optical method, a thickness variation is calculated from interference fringes, which occur between a referential plane of an optical lens and a surface of a wafer. The method enables high-speed measurement, but uses necessarily a vacuum chuck for attracting a backside surface of the wafer. When the wafer is attracted to a vacuum chuck inferior of flatness, the wafer itself is often deformed. The inferior flatness of the vacuum chuck is likely incorporated as an error in measurement results, so that the thickness variation of the wafer can not be detected with high reliability. There is also such the defect that chucking flaws are likely formed on the backside, since the wafer is held in direct contact with the vacuum chuck.
Use of interference fringes derived from light beams reflected on both surfaces of a wafer for detection of a thickness variation is disclosed in JP A 1-143906. In this method, light beams emitted from a light source are split to transmitting and reflecting beams by a beam splitter, reflected on both surfaces of a wafer and then inputted to light detectors. Optical interference fringes occur in correspondence with a difference in an optical path between the transmitting and reflecting beams.
On the other hand, in the physical method using displacement sensors, a deviation in a thickness of a wafer is calculated on the basis of positional signals detected by capacitance-type displacement sensors provided at both surfaces of the wafer, and a thickness variation of the wafer is calculated regarding its backside surface as an ideal flat plane. For instance, JP B 5-77179 discloses provision of displacement sensors at positions facing to both surfaces of a wafer, so as to produce deviation signals from every part of the wafer by rotation of the wafer.
The physical method using displacement sensors has been commonly used so far for detecting a thickness variation of a wafer, since measurement is performed with high reproductivity without any defects caused by a vacuum chuck. However, the wafer is necessarily rotated for scanning due to a small probe of the displacement sensor, so that it takes a longtime to scan a whole surface of the wafer.
When a surface part of the wafer attracted to a vacuum chuck is to be scanned, the wafer is re-held, and then motion of the sensors is changed to a swinging mode for scanning the remaining surface part which was attracted with the vacuum chuck, as disclosed in JP B 5-77179. The re-holding prolongs a measuring time in total and needs troublesome works.
Since the wafer is being rotated during measuring, both surfaces of the wafer come in contact with a large quantity of the air. Such contact means exposure of the wafer to adhesion of particles suspended in the air. Particles are also transferred from the vacuum chuck to the backside of the wafer. Adhesion of particles often unfavorably affects on measurement results.
When a wafer is scanned with displacement sensors, a circumferential part of the wafer is not subjected to scanning in order to avoid incorporation of edge effects into detected signals. In this regard, the circumferential part of the wafer has been regarded as an unmeasurable zone, resulting in reduction of a surface zone applicable for measurement of a thickness variation.
A wafer for measurement is horizontally held by attracting its backside center to a vacuum chuck. Due to this holding means, measurement results are likely affected by gravity as enlargement of the wafer in size. Deformation of the wafer at its periphery is often incorporated as an error into measurement results.
Defects caused by holding a wafer with a vacuum chuck is eliminated in an optical method of measuring a thickness variation of a wafer held in such a state kept free from a holding force, as disclosed in JP A 1-143906. According to this method, measurement is performed with ease in a short time, since a thickness variation is calculated from interference fringes, which occur between transmitting and reflecting light beams reflected on both surfaces of the wafer. Although the thickness variation is merely judged from the interference fringes, undulation or inclination of the wafer which is not accompanied with a thickness variation can not be detected. In addition, affections of particles floating in the air, positioning of a wafer, assembling accuracy of various members to a measuring apparatus, etc. are likely incorporated as errors into measurement results due to a long light path necessarily designed for occurrence of interference fringes.
In order to solve the above problems, the inventors proposed an optical apparatus for detecting profiles of a wafer from two sets of interference fringes, which occurred between optical flat lenses and S both surfaces of a wafer, as proposed in JP A 11-2512. In this apparatus, a pair of optical measuring systems are located at positions facing to both surfaces of a wafer which is vertically supported. The apparatus enables high-speed and accurate measurement of a thickness variation as well as profiles of both surfaces of a wafer without either adhesion of particles or formation of flaws.
Pair of flat lenses, which respectively face to main and backside surfaces of a wafer, must be located in a condition perfectly parallel each other, in order to produce interference fringes which accurately represent profiles of both surfaces of the wafer. However, the parallelism tends to be affected by fluctuation of room temperature, deformation of the lens supporting members in the laps of time, etc.
Poor parallelism is unfavorably incorporated as an error into calculation of a thickness variation, resulting in lack of reliabilities in measured data. If affection of the poor parallelism is eliminated, the optical measuring apparatus can be used for measuring a thickness variation and profiles of the both surfaces of a wafer with high accuracy and reliability.
The present invention is an improvement in the optical apparatus proposed in the former patent application (JP A 11-2512) for high-speed and precise measurement of wafers.
The object of the present invention is to enable high-speed and precise measurement of a real shape of a wafer including a thickness variation and profiles of both surfaces. The object is accomplished by calculation of profile data representing both surfaces of the wafer obtained by an optical measuring apparatus in consultation with thickness values actually detected by displacement sensor provided at several points of the wafer. The newly proposed method effectively deletes errors derived from poor parallelism between light paths of two optical measuring systems, due to consultation with actually detected thickness values.
The newly proposed optical profile measuring apparatus comprises a pair of optical measuring systems located at positions facing to both surfaces of a wafer vertically supported at its edge and a thickness thickness-measuring sensors located at a position or positions facing to a circumferential part of both surfaces of the wafer.
Each optical measuring system has a light source for emitting a measuring light beam, a collimator lens for rectifying the measuring light beam into a collimated beam, an optical flat for transmitting the collimated measuring beam therethrough, a light detector for receiving the measuring beams which have been reflected on a surface of said wafer and on a referential plane of said optical flat and then returned through the collimator lens, and a computer for processing interference fringes which occur between the two light beams reflected on the referential plane of the optical flat and the surface of the wafer. Profiles of both surfaces of the wafer are calculated from the interference fringes corresponding to both surfaces of the wafer, and a real shape of the wafer is grasped from the profiles in consultation with thickness values actually measured at a plurality of points of the wafer by the thickness gauge.
Triangular prisms may be used instead of optical flats. In this case, interference fringes corresponding to both surfaces of the wafer occur between referential planes of the triangular prisms and both surfaces of the wafer.